Articles

A series of 3,4,5-tridodecyloxybenzyl pyridinium salts derived from 4-hydroxypyridine has been designed and prepared. The liquid crystalline properties of these compounds were investigated by polarized optical microscopy, differential scanning calorimetry and powder X-ray diffraction while their thermal stability was studied by thermogravimetric analysis. The N-3,4,5-tridodecyloxybenzyl-4-pyridone intermediate shows a monotropic columnar hexagonal mesophase ranging from 56 C down to room temperature while the corresponding bromide dodecyl O-alkylated pyridinium salt shows one enantiotropic columnar mesophase and one additional monotropic columnar phase at lower temperatures. Replacing bromide ion (Br ) with other counterions (NO 3 ; BF 4 and PF 6 ) resulted in mesophase suppression. These luminescent pyridinium salts show weak emission in dichloromethane solutions at room temperature and a pronounced red-shifted emission in solid state. Photoluminescent properties of the pyridinium salts do not depend significantly on the nature of counterion employed.

A new class of thermotropic lanthanido mesogens has been designed and prepared. They are based on 4-pyridone ligands that possess mesogenic cyanobiphenyl groups attached to the 4-pyridone unit via a flexible long alkyl spacer and show a very high thermal stability (decomposition temperatures near 300 °C). Depending on the alkyl length spacer, these complexes exhibit a SmA phase with transition temperatures influenced by the number of mesogenic groups employed and the spacer length.

A series of novel liquid crystals with various lanthanide ions (Eu(III), Sm(III) and Tb(III)) was designed and prepared starting from corresponding lanthanide nitrates and N-alkylated 4-pyridones derivatives with mesogenic 3,4,5-tris(alkyloxy)benzyl moieties (alkyl= hexyl, octyl, decyl, dodecyl, tetradecyl or hexadecyl). These new lanthanidomesogens were investigated for their mesogenic properties by a combination of differential scanning calorimetry (DSC), polarizing optical microscopy (POM) and temperature dependent powder X-ray diffraction (XRD). Additionally, their thermal stability was assessed by thermogravimetric analysis (TG). All these complexes display an enantiotropic liquid crystalline behavior with either a lamellar (SmA), in the case of shorter chains analogues (6 and 8 carbon atoms) or a hexagonal columnar (Colh) phase, for complexes with higher number of carbon atoms (12, 14 and 16), assigned on the basis of their characteristic texture and XRD studies. For complexes with an intermediate number of carbon atoms in the side chains (10), both a lamellar phase at lower temperatures and a Colh phase at higher temperatures were evidenced. In solid state, all these complexes show characteristic emissions assigned to the corresponding lanthanide ion. In addition, the luminescence decay curves showed single exponential decays with the characteristic times in the ms range (0.75-0.90 ms for Eu(III), 0.045-0.060 ms for Sm(III) and 0.75-1.05 ms for Tb(III), respectively).